Recently, in the field of a color display device used for image display on a computer or a television, a plasma display device including a plasma display panel (PDP) receives attention because it can be enlarged, thinned, and lightened. The plasma display device displays an image in full color by additive mixing of so-called three primary colors (red, green, blue). For performing the full color display, the plasma display device has a phosphor layer for emitting light of each of three primary colors, red (R), green (G), and blue (B), and phosphor particles constituting the phosphor layer are excited by ultraviolet rays generated in a discharge cell of the PDP and hence generate visible light of each color.
A compound is used for a phosphor of each color, the compound for emitting red light is. (YGd)BO3:Eu3+ or Y2O3:Eu3+, the compound for emitting green light is Zn2SiO4:Mn2+, and the compound for emitting blue light is BaMgAl10O17:Eu2+, for example. These phosphors are produced generally by mixing predetermined raw materials and sintering them at a temperature of 1000° C. or higher for solid phase reaction. Phosphor particles obtained by the sintering are further crushed and classified to produce phosphor particles having a predetermined grain size. For example, an average grain size of red and green phosphor particles is 2 μm to 5 μm, and an average grain size of blue phosphor particles is 3 μm to 10 μm. The reason why the phosphor particles are crushed and classified is described below. For forming a phosphor layer on the PDP, generally, a screen-printing method of pasting phosphor particles of each color, or an ink jet method of delivering phosphor ink through a narrow nozzle is used. In these method, as the phosphor has smaller and more uniform grain size, namely as grain size distribution is more uniform, a smoother coated surface is easily formed in pasting. In other words, as the phosphor has smaller and more uniform grain size and has a shape closer to sphere, the coated surface is smoother, filling density of the phosphor particles in the phosphor layer increases, light emitting surface area of the particles increases, and unstability in address driving improves. Luminance of the plasma display device can be therefore increased.
When the grain size of the phosphor particles is decreased, however, the surface area of the particles increases and a defect in the phosphor is apt to increase. Thus, much water, carbon dioxide, or hydrocarbon-based organic matters are apt to adsorb onto a surface of the phosphor. Especially, in the blue phosphor such as Ba1−XMgAl10O17:EuX, CaMgSi2O6:Eu, or Ca3MgSi2O8:Eu in which bivalent europium (Eu) ions mainly emit light, originally stable trivalent Eu is reduced to bivalent europium, so that an oxygen defect occurs in a crystal during reduction. It is especially shown that increasing the substitution ratio of Eu increases the oxygen defect amount. Water and carbon hydride existing in the air thus, selectively adsorb to an oxygen defect near calcium (Ca), strontium (Sr), barium (Ba), or Eu ion in a phosphor crystal. Therefore, in a panel manufacturing process, much water and carbon hydride are released into a discharge space of the panel and react with the phosphor and magnesium oxide (MgO) functioning as protective film during the discharge. A phenomenon such as luminance degradation, chromaticity change (color shift or screen seizure due to chromaticity change), decrease of driving margin, or increase of discharge voltage is apt to occur.
Since water and hydrocarbon gas selectively adsorb to a blue phosphor, ethylcellulose in a binder hardly adsorbs to the blue phosphor in producing paste or ink, and hence the blue phosphor is apt to separate from the ethylcellulose. When the ethylcellulose and the blue phosphor are separated from each other, the blue phosphor is apt to accumulate near a nozzle opening having zero velocity gradient especially in the inkjet method, and hence the nozzle is disadvantageously clogged.
A method of addressing these problems by coating the entire surface of the phosphor with alumina (Al2O3) film is disclosed, for example, in Japanese Patent Unexamined Publication No. 2001-55567. The coating of the entire surface newly causes absorbing of ultraviolet rays to decrease light emitting luminance of the phosphor, and the coating cannot sufficiently prevent the decrease of luminance due to the ultraviolet rays.
The phosphor used in a PDP or the like is manufactured by a solid-phase reaction method or an aqueous solution reaction method, but decreasing of the grain size is apt to generate a defect. In the solid-phase reaction method, especially, sintering or crushing the phosphor in reducing atmosphere is known to generate many defects. Ultraviolet rays having 147 nm of wavelength generated by discharge in driving the panel is also known to generate a defect in the phosphor (for example, Institute of Electronics, Information and Communication, Technology and research report, EID99-94 Jan. 27, 2000).
Especially, the blue phosphor such as BaMgAl10O17:Eu or Ca3MgSi2O8:Eu where bivalent Eu ions emit light is known to include an oxygen defect in the phosphor material itself (for example, Applied physics, Vol. 70, No. 3, 2001, PP310).
The generation of these defects is considered to cause luminance degradation of the conventional blue phosphor by itself. In other words, it has been considered that a defect generated by an impact on the phosphor from an ion occurring in driving the panel or a defect generated by the ultraviolet rays having 147 nm of wavelength causes the luminance degradation.